Cosmology
Recombination Era
When the universe became transparent — electrons captured by protons, ~380,000 years after BB
The recombination era is when the universe transitioned from opaque plasma to transparent gas — when electrons combined with protons to form neutral hydrogen atoms. Occurred ~380,000 years after Big Bang at temperature ~3000 K. Photons no longer scattered off free electrons → free-streamed → CMB. Surface of last scattering — boundary in cosmic time. Before: opaque, electromagnetic interactions limit observation. After: transparent, photons reach us today as 2.725 K CMB. Critical moment in cosmic history.
- Occurred at~380,000 years after BB
- Temperature~3000 K (when atoms could form)
- Universe size~1100× smaller than today
- Hydrogen ionizationGoes from 100% ionized to mostly neutral
- DefinesSurface of last scattering (CMB)
- Mechanisme⁻ + p → H + photon (recombination)
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Why recombination matters
- CMB origin. Where photons came from.
- Cosmology. Defines surface of last scattering.
- Universe transparency. Marker of cosmic transition.
- Structure formation. After recombination, baryons clump.
- Big Bang confirmation. Matches theoretical prediction.
- Atomic physics. Ionization energy 13.6 eV → recombination T.
- Cosmological time. Reference point in early universe.
Common misconceptions
- Recombination is recombining. First combination — name historical.
- It happened immediately after BB. 380,000 years later.
- Universe was hot until recombination. Cooling continuously.
- Photons came from BB itself. Photons released at recombination.
- Hydrogen formed only at recombination. Same; before it was ionized.
- Recombination was instantaneous. ~50,000 yr to complete.
Frequently asked questions
Why is it called "recombination"?
Misleading name. Atoms forming for first time (not "re-"). Term arose because process is the inverse of ionization — photoelectrons recombining with ions. Standard physics terminology. Some prefer "primordial recombination" or "hydrogen formation." Commonly called recombination.
Why ~380,000 years?
Universe cooling as it expanded. At T ~3000 K, photons no longer have enough energy to ionize hydrogen. Bound atoms become stable. T = 3000 K corresponds to ~380,000 yr after BB. Specific T comes from atomic physics — ionization energy of hydrogen (13.6 eV).
What was before recombination?
Hot plasma. Free electrons + protons + photons + dark matter. Opaque to light. Photons scatter off electrons (Thomson scattering). Universe optically thick. Sound waves propagate through plasma — "baryonic acoustic oscillations" leave imprint on CMB power spectrum.
Why does CMB exist?
Photons released at recombination free-stream until reaching us today. Thermal radiation from when universe was 3000 K → stretched by 1100× redshift → observed as 2.725 K microwave. Sky uniformly bathed in this primordial light. Detected 1964; mapped by COBE, WMAP, Planck.
Did everything recombine at once?
Process took ~50,000 years to complete. Started ~370,000 yr (slight ionization remaining). Completed ~420,000 yr. Surface of last scattering has finite thickness. Anisotropies in CMB include effects from this thickness.
What about dark matter at recombination?
Dark matter doesn't interact with photons — already free-streaming. Dark matter clumps before recombination (during radiation era). Then ordinary matter (baryons) joins dark matter clumps after recombination — gravity dominant. Sets up structure formation.
How is recombination tested?
(1) CMB temperature 2.725 K matches prediction. (2) CMB anisotropy structure depends on recombination physics. (3) Light element abundances (BBN) require thermal history through recombination. (4) BBN happens earlier (3 min); recombination much later. All consistent.